In recent years, there has been an increasing demand for a high-strength steel sheet used in a vehicle or the like, and a high-strength cold-rolled steel sheet with a maximum tensile stress of 900 MPa or more is also being used.
Generally, as the strength of a steel sheet is enhanced, ductility and stretch-flangeability are lowered, and workability is degraded. However, a high-strength steel sheet with sufficient workability has been demanded in recent years.
As a conventional technique for enhancing ductility and stretch-flangeability of a high-strength steel sheet, a high-tensile galvanized steel sheet, which has a composition containing by mass percentage, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, S: 0.005% or less, the remainder composed of Fe and inevitable impurities, has a composite structure including ferrite, tempered martensite, retained austenite, and low temperature transformation phase, and contains by volume percentage 30% or more of ferrite, 20% or more of tempered martensite, 2% or more of retained austenite, in which average crystal grain sizes of ferrite and tempered martensite are 10 μm or less, is an exemplary example (see Patent Document 1, for example).
In addition, as a conventional technique for enhancing workability of a high-strength steel sheet, a high-tensile cold-rolled steel sheet, in which amounts of C, Si, Mn, P, S, Al, and N are adjusted, which further contains 3% or more of ferrite and a total of 40% or more of bainite containing carbide and martensite containing carbide as metal structures of the steel sheet containing one or more of Ti, Nb, V, B, Cr, Mo, Cu, Ni, and Ca as necessary, in which the total amount of ferrite, bainite, and martensite is 60% or more, and which further has a structure in which the number of ferrite grains containing cementite, martensite, or retained austenite therein corresponds to 30% or more of the total number of ferrite grains and has tensile strength of 780 MPa or more, is an exemplary example (see Patent Document 2, for example).
Moreover, as a conventional technique for enhancing stretch-flangeability of a high-strength steel sheet, a steel sheet in which a difference in hardness between a hard part and a soft part of the steel sheet is reduced is an exemplary example. For example, Patent Document 3 discloses a technique in which the standard deviation of hardness in the steel sheet is reduced and uniform hardness is given to the entire steel sheet. Patent Document 4 discloses a technique in which hardness in the hard part is lowered by heat treatment and the difference in hardness from that in the soft part is reduced. Patent Document 5 discloses a technique in which the difference in hardness from the soft part is reduced by configuring the hard part of relatively soft bainite.
Furthermore, as a conventional technique for enhancing stretch-flangeability of a high-strength steel sheet, a steel sheet, which has a structure containing by an area ratio 40 to 70% of tempered martensite and a remainder composed of ferrite, in which a ratio between an upper limit value and a lower limit value of Mn concentration in a cross-section in a thickness direction of the steel sheet is reduced (see Patent Document 6, for example) may be exemplified.